This invention relates to meta-halogenated phenols and to processes for their preparation.
Halogenated phenols are often used as fungicides, monomers and flame retardants. Certain halogenated phenols such as polybrominated phenols are used in the manufacture of brominated aromatic epoxy thermosets which are especially useful in the electronics industry.
Such polybrominated phenols are usually prepared by contacting a phenol such as bisphenol-A with bromine in methanol or aqueous acetic acid thereby producing the ortho-brominated phenols such as tetrabromobisphenol-A. However, the hydrolytic instability of the ortho-bromophenols has been observed to contribute to internal corrosion of the encapsulated parts of many electronic devices, thereby reducing or destroying the effectiveness of the electronic device employing the encapsulated parts, e.g., microelectric circuitry.
As reported in U.S. Pat. No. 4,731,423, the metabromophenols are known to be more hydrolytically stable than their ortho-bromo counterparts. It is, therefore, particularly desirable to employ such meta-bromophenols in epoxy thermosets used in electrical encapsulations which require a thermoset having good thermal properties, e.g., flame retarding capability and glass transition temperatures (T.sub.g) of at least 155.degree. C. as well as hydrolytic stability significantly greater than that observed for the ortho-bromophenols. A major reason for the need for higher thermal stability is the miniaturization in the art of microelectric circuitry, often called "microchip" technology. As the size of the microchips becomes increasingly smaller, the localized heat problems become increasingly greater, and thus the present encapsulation formulation properties may not be as suitable as desired.
Unfortunately, the thermally and hydrolytically stable meta-bromophenols are often difficult to prepare and yields of the desired products are usually only modest. See, for example, the methods described in U.S. Pat. Nos. 3,929,908: 3,956,403: and 4,058,570. Moreover, the methods for making the simple meta-bromophenols, which usually involve the bromination of the corresponding phenol, are essentially useless for the preparation of complex meta-bromophenols because the complex phenol precursors are decomposed upon bromination. Examples of such complex phenols are the dibenzylphenols as described in U.S. Pat. No. 4,222,884.
Therefore it would be highly desirable to provide a process for making complex meta-halophenols that cannot be made by typical bromination or chlorination processes.